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mm/page_counter: move calculating protection values to page_counter
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It's a lot of math, and there is nothing memcontrol specific about it. 
This makes it easier to use inside of the drm cgroup controller.

[[email protected]: fix kerneldoc, per Jeff Johnson]
Link: https://lkml.kernel.org/r/[email protected]
Signed-off-by: Maarten Lankhorst <[email protected]>
Acked-by: Roman Gushchin <[email protected]>
Acked-by: Shakeel Butt <[email protected]>
Cc: Michal Hocko <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Muchun Song <[email protected]>
Cc: Jeff Johnson <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
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@mlankhorst @akpm00
mlankhorst authored and akpm00 committed Jul 12, 2024
1 parent 3b0ba54 commit a8585ac
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Showing 3 changed files with 180 additions and 151 deletions.
4 changes: 4 additions & 0 deletions include/linux/page_counter.h
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Original file line number Diff line number Diff line change
Expand Up @@ -81,4 +81,8 @@ static inline void page_counter_reset_watermark(struct page_counter *counter)
counter->watermark = page_counter_read(counter);
}

void page_counter_calculate_protection(struct page_counter *root,
struct page_counter *counter,
bool recursive_protection);

#endif /* _LINUX_PAGE_COUNTER_H */
154 changes: 3 additions & 151 deletions mm/memcontrol.c
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Original file line number Diff line number Diff line change
Expand Up @@ -4390,122 +4390,6 @@ struct cgroup_subsys memory_cgrp_subsys = {
.early_init = 0,
};

/*
* This function calculates an individual cgroup's effective
* protection which is derived from its own memory.min/low, its
* parent's and siblings' settings, as well as the actual memory
* distribution in the tree.
*
* The following rules apply to the effective protection values:
*
* 1. At the first level of reclaim, effective protection is equal to
* the declared protection in memory.min and memory.low.
*
* 2. To enable safe delegation of the protection configuration, at
* subsequent levels the effective protection is capped to the
* parent's effective protection.
*
* 3. To make complex and dynamic subtrees easier to configure, the
* user is allowed to overcommit the declared protection at a given
* level. If that is the case, the parent's effective protection is
* distributed to the children in proportion to how much protection
* they have declared and how much of it they are utilizing.
*
* This makes distribution proportional, but also work-conserving:
* if one cgroup claims much more protection than it uses memory,
* the unused remainder is available to its siblings.
*
* 4. Conversely, when the declared protection is undercommitted at a
* given level, the distribution of the larger parental protection
* budget is NOT proportional. A cgroup's protection from a sibling
* is capped to its own memory.min/low setting.
*
* 5. However, to allow protecting recursive subtrees from each other
* without having to declare each individual cgroup's fixed share
* of the ancestor's claim to protection, any unutilized -
* "floating" - protection from up the tree is distributed in
* proportion to each cgroup's *usage*. This makes the protection
* neutral wrt sibling cgroups and lets them compete freely over
* the shared parental protection budget, but it protects the
* subtree as a whole from neighboring subtrees.
*
* Note that 4. and 5. are not in conflict: 4. is about protecting
* against immediate siblings whereas 5. is about protecting against
* neighboring subtrees.
*/
static unsigned long effective_protection(unsigned long usage,
unsigned long parent_usage,
unsigned long setting,
unsigned long parent_effective,
unsigned long siblings_protected)
{
unsigned long protected;
unsigned long ep;

protected = min(usage, setting);
/*
* If all cgroups at this level combined claim and use more
* protection than what the parent affords them, distribute
* shares in proportion to utilization.
*
* We are using actual utilization rather than the statically
* claimed protection in order to be work-conserving: claimed
* but unused protection is available to siblings that would
* otherwise get a smaller chunk than what they claimed.
*/
if (siblings_protected > parent_effective)
return protected * parent_effective / siblings_protected;

/*
* Ok, utilized protection of all children is within what the
* parent affords them, so we know whatever this child claims
* and utilizes is effectively protected.
*
* If there is unprotected usage beyond this value, reclaim
* will apply pressure in proportion to that amount.
*
* If there is unutilized protection, the cgroup will be fully
* shielded from reclaim, but we do return a smaller value for
* protection than what the group could enjoy in theory. This
* is okay. With the overcommit distribution above, effective
* protection is always dependent on how memory is actually
* consumed among the siblings anyway.
*/
ep = protected;

/*
* If the children aren't claiming (all of) the protection
* afforded to them by the parent, distribute the remainder in
* proportion to the (unprotected) memory of each cgroup. That
* way, cgroups that aren't explicitly prioritized wrt each
* other compete freely over the allowance, but they are
* collectively protected from neighboring trees.
*
* We're using unprotected memory for the weight so that if
* some cgroups DO claim explicit protection, we don't protect
* the same bytes twice.
*
* Check both usage and parent_usage against the respective
* protected values. One should imply the other, but they
* aren't read atomically - make sure the division is sane.
*/
if (!(cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT))
return ep;
if (parent_effective > siblings_protected &&
parent_usage > siblings_protected &&
usage > protected) {
unsigned long unclaimed;

unclaimed = parent_effective - siblings_protected;
unclaimed *= usage - protected;
unclaimed /= parent_usage - siblings_protected;

ep += unclaimed;
}

return ep;
}

/**
* mem_cgroup_calculate_protection - check if memory consumption is in the normal range
* @root: the top ancestor of the sub-tree being checked
Expand All @@ -4517,48 +4401,16 @@ static unsigned long effective_protection(unsigned long usage,
void mem_cgroup_calculate_protection(struct mem_cgroup *root,
struct mem_cgroup *memcg)
{
unsigned long usage, parent_usage;
struct mem_cgroup *parent;
bool recursive_protection =
cgrp_dfl_root.flags & CGRP_ROOT_MEMORY_RECURSIVE_PROT;

if (mem_cgroup_disabled())
return;

if (!root)
root = root_mem_cgroup;

/*
* Effective values of the reclaim targets are ignored so they
* can be stale. Have a look at mem_cgroup_protection for more
* details.
* TODO: calculation should be more robust so that we do not need
* that special casing.
*/
if (memcg == root)
return;

usage = page_counter_read(&memcg->memory);
if (!usage)
return;

parent = parent_mem_cgroup(memcg);

if (parent == root) {
memcg->memory.emin = READ_ONCE(memcg->memory.min);
memcg->memory.elow = READ_ONCE(memcg->memory.low);
return;
}

parent_usage = page_counter_read(&parent->memory);

WRITE_ONCE(memcg->memory.emin, effective_protection(usage, parent_usage,
READ_ONCE(memcg->memory.min),
READ_ONCE(parent->memory.emin),
atomic_long_read(&parent->memory.children_min_usage)));

WRITE_ONCE(memcg->memory.elow, effective_protection(usage, parent_usage,
READ_ONCE(memcg->memory.low),
READ_ONCE(parent->memory.elow),
atomic_long_read(&parent->memory.children_low_usage)));
page_counter_calculate_protection(&root->memory, &memcg->memory, recursive_protection);
}

static int charge_memcg(struct folio *folio, struct mem_cgroup *memcg,
Expand Down
173 changes: 173 additions & 0 deletions mm/page_counter.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -262,3 +262,176 @@ int page_counter_memparse(const char *buf, const char *max,

return 0;
}


/*
* This function calculates an individual page counter's effective
* protection which is derived from its own memory.min/low, its
* parent's and siblings' settings, as well as the actual memory
* distribution in the tree.
*
* The following rules apply to the effective protection values:
*
* 1. At the first level of reclaim, effective protection is equal to
* the declared protection in memory.min and memory.low.
*
* 2. To enable safe delegation of the protection configuration, at
* subsequent levels the effective protection is capped to the
* parent's effective protection.
*
* 3. To make complex and dynamic subtrees easier to configure, the
* user is allowed to overcommit the declared protection at a given
* level. If that is the case, the parent's effective protection is
* distributed to the children in proportion to how much protection
* they have declared and how much of it they are utilizing.
*
* This makes distribution proportional, but also work-conserving:
* if one counter claims much more protection than it uses memory,
* the unused remainder is available to its siblings.
*
* 4. Conversely, when the declared protection is undercommitted at a
* given level, the distribution of the larger parental protection
* budget is NOT proportional. A counter's protection from a sibling
* is capped to its own memory.min/low setting.
*
* 5. However, to allow protecting recursive subtrees from each other
* without having to declare each individual counter's fixed share
* of the ancestor's claim to protection, any unutilized -
* "floating" - protection from up the tree is distributed in
* proportion to each counter's *usage*. This makes the protection
* neutral wrt sibling cgroups and lets them compete freely over
* the shared parental protection budget, but it protects the
* subtree as a whole from neighboring subtrees.
*
* Note that 4. and 5. are not in conflict: 4. is about protecting
* against immediate siblings whereas 5. is about protecting against
* neighboring subtrees.
*/
static unsigned long effective_protection(unsigned long usage,
unsigned long parent_usage,
unsigned long setting,
unsigned long parent_effective,
unsigned long siblings_protected,
bool recursive_protection)
{
unsigned long protected;
unsigned long ep;

protected = min(usage, setting);
/*
* If all cgroups at this level combined claim and use more
* protection than what the parent affords them, distribute
* shares in proportion to utilization.
*
* We are using actual utilization rather than the statically
* claimed protection in order to be work-conserving: claimed
* but unused protection is available to siblings that would
* otherwise get a smaller chunk than what they claimed.
*/
if (siblings_protected > parent_effective)
return protected * parent_effective / siblings_protected;

/*
* Ok, utilized protection of all children is within what the
* parent affords them, so we know whatever this child claims
* and utilizes is effectively protected.
*
* If there is unprotected usage beyond this value, reclaim
* will apply pressure in proportion to that amount.
*
* If there is unutilized protection, the cgroup will be fully
* shielded from reclaim, but we do return a smaller value for
* protection than what the group could enjoy in theory. This
* is okay. With the overcommit distribution above, effective
* protection is always dependent on how memory is actually
* consumed among the siblings anyway.
*/
ep = protected;

/*
* If the children aren't claiming (all of) the protection
* afforded to them by the parent, distribute the remainder in
* proportion to the (unprotected) memory of each cgroup. That
* way, cgroups that aren't explicitly prioritized wrt each
* other compete freely over the allowance, but they are
* collectively protected from neighboring trees.
*
* We're using unprotected memory for the weight so that if
* some cgroups DO claim explicit protection, we don't protect
* the same bytes twice.
*
* Check both usage and parent_usage against the respective
* protected values. One should imply the other, but they
* aren't read atomically - make sure the division is sane.
*/
if (!recursive_protection)
return ep;

if (parent_effective > siblings_protected &&
parent_usage > siblings_protected &&
usage > protected) {
unsigned long unclaimed;

unclaimed = parent_effective - siblings_protected;
unclaimed *= usage - protected;
unclaimed /= parent_usage - siblings_protected;

ep += unclaimed;
}

return ep;
}


/**
* page_counter_calculate_protection - check if memory consumption is in the normal range
* @root: the top ancestor of the sub-tree being checked
* @counter: the page_counter the counter to update
* @recursive_protection: Whether to use memory_recursiveprot behavior.
*
* Calculates elow/emin thresholds for given page_counter.
*
* WARNING: This function is not stateless! It can only be used as part
* of a top-down tree iteration, not for isolated queries.
*/
void page_counter_calculate_protection(struct page_counter *root,
struct page_counter *counter,
bool recursive_protection)
{
unsigned long usage, parent_usage;
struct page_counter *parent = counter->parent;

/*
* Effective values of the reclaim targets are ignored so they
* can be stale. Have a look at mem_cgroup_protection for more
* details.
* TODO: calculation should be more robust so that we do not need
* that special casing.
*/
if (root == counter)
return;

usage = page_counter_read(counter);
if (!usage)
return;

if (parent == root) {
counter->emin = READ_ONCE(counter->min);
counter->elow = READ_ONCE(counter->low);
return;
}

parent_usage = page_counter_read(parent);

WRITE_ONCE(counter->emin, effective_protection(usage, parent_usage,
READ_ONCE(counter->min),
READ_ONCE(parent->emin),
atomic_long_read(&parent->children_min_usage),
recursive_protection));

WRITE_ONCE(counter->elow, effective_protection(usage, parent_usage,
READ_ONCE(counter->low),
READ_ONCE(parent->elow),
atomic_long_read(&parent->children_low_usage),
recursive_protection));
}

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